A lighting lamp such as a fluorescent lamp, etc., has a definite lamp life and since a lighting lamp the life of which has been gone must be renewed, it has been desired to detect the lamp life of a lighting lamp before the termination of the life of the lighting lamp for renewing. Hitherto, the lamp life of a lighting lamp is determined by observing the blackened phenomenon at the end portion of the bulb.
Also, it is known to apply an organic material such as a coating material, etc., capable of being discolored or faded by the action of ultraviolet rays to the surface of a bulb of a lighting lamp and determine the lamp life by the extent of the discoloration or the fading as disclosed in JP-A-U-53-14057 and JP-A-U-62-133366. (The term "JP-A-U" as used herein means an "unexamined published Japanese utility model application".)
Now, the blackened phenomenon at the end portion of a bulb of a lighting lamp differs according to the using condition of the lighting lamp and hence it is difficult to accurately determine the lamp life by the blackened phenomenon. Also, when an organic material discoloring or fading by the action of ultraviolet rays is applied to the surface of a bulb of a lighting lamp, the organic material is influenced by the light of other lighting lamps or external light and hence the extent of discoloring or fading changes or the control of the extent of discoloring or fading is not always easy as well as the final state of discoloring or fading is indistinct. Accordingly, there is a problem that the accurate determination of the lamp life is difficult by such a conventional technique.